In recent years, films (sheets are also included. The same definition will apply to the following description) are used in the field of food packages such as trays and ice cream cups. Further, films are also used in the field of drug packages such as PTP drug packages for capsules and tablets. Further, films are also used in the field of laminated molded articles used in furniture, indoor/outdoor ornaments, electrical appliances and automobile parts (for example, a laminated molded article comprising a film and paper, wood, metal or a resin). Further, films are also used in the field of materials for cards (e.g., IC cards and magnetic recording cards) such as ATM cards, ID cards and credit cards. Further, films are also used in the field of agriculture such as a green house. As described above, films are used in a wide variety of fields.
In general, these materials are often products having nonflat surfaces such as curved surfaces and uneven surfaces in addition to products having flat surfaces. Accordingly, a rigid polyvinyl chloride resin film having excellent processability and moldability is primarily used. However, since the polyvinyl chloride resin has poor heat resistance, a product using the resin may be deformed when exposed to high temperatures, e.g., left in an automobile in midsummer. Further, the polyvinyl chloride resin causes an environmental problem since it is liable to generate chlorine-based harmful substances when burned after used.
Further, an unoriented polyethylene terephthalate film called “A-PET” (trade name) is also used. However, although this has excellent moldability, it has a problem that when it is used over a long time period, an embrittlement phenomenon occurs, and it undergoes a loss of transparency and a decrease in elongation. To solve this problem, a method using an unoriented thermally crystallized polyethylene terephthalate film is proposed (Patent Publication 1). However, this film has a problem that it has poor embossing processability and moldability.
Further, a 1,4-cyclohexane dimethanol derivative copolymerized polyester is proposed as a substitute resin for a polyvinyl chloride. However, although a film of this resin shows improvements in moldability and a change with time, the polymer on the surface of the film is dissolved by a solvent in ink and the ink is liable to set off when printing is made on the film by use of ink or the like. That is, this film has poor organic solvent resistance. Further, a product made of the film is liable to be curled or deformed, indicating that the film has insufficient dimensional stability.
To solve these problems, the present inventor has previously proposed a multilayer polyester film comprising a polyester layer having an oriented structure and a polyester layer having a substantially unoriented structure (Patent Publications 2 and 3). However, it has been revealed that the moldability of the multilayer polyester film still needs to be improved.
That is, it has been revealed that when a molded article with an uneven structure is produced from the multilayer polyester film, it shows better moldability than the conventional unoriented thermally crystallized polyethylene terephthalate film, but the shape of the molded article is liable to be distorted. It is assumed that the distortion is caused by unevenness in thickness due to unsatisfactory in-plane isotropy of the film.
(Patent Publication 1)
Japanese Patent Laid-Open Publication No. 8-279150
(Patent Publication 2)
Japanese Patent Laid-Open Publication No. 2002-96438
(Patent Publication 3)
Japanese Patent Laid-Open Publication No. 2002-96439